The present invention generally relates to devices and methods for flow measurement of fluids produced from hydrocarbon wells. More particularly, embodiments of the present invention provide an apparatus and method for obtaining a composite sample from a well in a production test mode, where the composite sample is obtained incrementally over the duration of the production test. Having the composite sample allows a precise determination of the water cut through conventional volumetric measurement methods. Once a precise water cut is determined, an oil producer can determine an accurate net oil production rate for a well. Among other uses, the observed water cut information may be utilized to accurately determine the economic limit of the well, to evaluate the success of a well treatment program, and/or to program various devices which require input of water cut information.
The water cut measurement—the percentage of water in a liquid production stream—is the most important factor in accurately determining the volume of net oil produced by an oil well. Determination of the water cut may be accomplished in a variety of ways. Some production facilities utilize three-phase flow separation systems which first separate the gas phase from the liquid phase, and then separate the liquid phase into an oil phase and a water phase and meter each liquid stream. More typically, because of the expense of three-phase flow separation systems and the retention time which may be required to sufficiently separate the oil phase from the water phase, two-phase systems are more commonly utilized. With these systems, the gas phase is separated from the liquid phase but there is no separation of the oil and water in the liquid phase. The liquid phase must then be analyzed either manually or with analytical devices to determine the water cut.
Water cut meters are the typical analytical devices utilized to determine water cut. These devices measure a liquid phase flow and then ascertain the relative percentage of water in the flow stream utilizing a variety of technologies, such as dielectric measurement (capacitance), microwave, infrared. However, utilizing water cut meters to determine net oil becomes more uncertain as the water cut increases. As the water cut in the liquid stream increases, the net oil measurement of the known water cut meters loses accuracy. In such cases, obtaining a reliable water cut measurement requires sampling the flow stream and directly measuring the percentage of water in the sample. However, such sampling is typically done by opening a valve for a brief period of time and “grabbing” a sample. However, because of changing flow conditions and flow regimes, a grabbed sample does not necessarily provide a liquid sample which is representative of the well's production over time.
Because net oil production is determined from the water cut, accurate determination of the water cut is an important factor in determining the economic viability of a well or field. Unfortunately, at the time when this information is perhaps needed the most, the accuracy of water cut data is likely to be the most difficult to achieve because an increase in water cut is common for mature, depleting water drive reservoirs. An apparatus which results in the well test systems providing accurate water cut measurement, particularly where the production stream has a high water cut, is desirable.